Patent GB-255163-A (GB)
Title: Improvements in or relating to wireless and like receiving systems provided with sound-filtering devices
IPC: H04B1/10
Filed: 1925-04-18
Abstract: 255,163. Roos, O. C. April 18, 1925. Atmospherics and undesired signals, eliminating; tone or group frequency itining-arrangements.-The energy due to shock excitation by atmospherics is converted into stationary waves of definite frequency in a closed tubular resonator or the like; the heterodyne beat note is adjusted to a harmonic or other simply related frequency, and the two are then separated by means of acoustic wave-filters. The pitch allotted to the shock note is preferably that of the most prominent interfering note arising from undesired signals, so that the two may be suppressed together. The resonators may contain liquid media instead of air. General arrangement. The loud-speaker telephone 1, Fig. 1, when affected by an atmospheric, gives rise to a shock note in the " percussion chamber " 2, setting up stationary waves in the resonator or " echo chamber " 9. Similar waves, of different frequency, are set up by the desired signal note, their pressure nodes being differently located from those of the shock note (" dephased "). The end 15 of a telescopic tube 11, 12, tuned to the signal note, is located at a pressure node of the desired waves; this tube communicates with a further filter chamber 10. The desired pressure variations are then applied to one focus of the ellipsoidal or paraboloidal " reflection absorption chamber " 28, at the other focus of which is a telephonic detector 40. The signals are observed, after amplification, in a telephone T shunted by an audibility meter N. As an alternative to the above arrangements, the resonator 9 or 10 may be selectively tapped by means of " counterphase tubes " such as 50, 51, Fig. 11, using the principle of phase interference. The resonators mav be shielded from external sound by enclosing them in felt-lined containers or the like. Percussion chambers. The pitch of the note produced by shock excitation of the loud-speaker is determined by the percussion chamber 2, Fig. 1. This may alternatively take the form of a short tube comprising a dowel 45, Fig. 20, which may be formed in two axially adjustable parts (not shown). There may be a gap open to the air between the loud-speaker and the percussion chamber, or the loud-speaker may speak directly into an open chamber or through adjustable tubes 89, 90, into two open chambers 93, 94, Fig. 20, the length of each chamber being a quarter of the wavelength of the note to be suppressed. Selective tapping of resonators. The desired heterodyne signal note is adjusted until its frequency bears a simple relation to that of the shock note to be suppressed; in Figs. 1 and 4 the signal note is the octave of the shock note. The lengths of the chambers are so arranged that, as illustrated in Fig. 4, the signal note has a pressure loop Pn, while the shock note has a pressure node Pn<1> at the middle of the chamber 9. The end 15 of the coupling tube 11, 12, is therefore located at this point, while the other end 16 is located at the end of the next chamber 10, where both notes have pressure loops. The input end 32 of the next coupling tube is similarly located at a point where only the signal will be selected. The telescopic coupling tube 11, 12, which is tuned to the signal frequency, is formed at its ends with lateral holes partially and adjustably covered by discs 17, 18, 19, 20. By narrowing the gaps between the discs selected can be increased at the expense of intensity. A damping partition 27, comprising a sheet of felt or the like or a wooden disc having holes filled with sound-absorbing material, is placed at some point where the signal wave has a velocity node while the shock wave has a velocity loop, or the resonators may be internally constricted by shaped hollow plugs at the pressure nodes of the signal. In the arrangement shown in Fig. 11 the shock note is the octave of the signal note. Tuned coupling tubes 50, 51, lead to a chamber 56 from points in the resonator 47 so chosen that the shock disturbances interfere and produce no variation of pressure in the chamber 56, while the signal disturbances produce a maximum variation. One of the tubes may be constricted to allow for inequality of the disturbances therein. A number of variations in the details of the construction are described in the Specification, and several different ratios of shock frequency to signal frequency are discussed. Toroidal filter chamber. Certain frequencies may be eliminated by admitting sound from the percussion chamber 45, Fig. 22, through tubes 99, 102, to a number of equally spaced points in an adjustable toroidal chamber 98, and tapping off the sound through tubes 104. The peripheral length of the ring is double the wave-length of the sound to be eliminated so that certain frequencies, including the fundamental and the suboctave, will be balanced out and will have no effect. Ellipsoidal or paraboloidal chambers for selective absorption of sound. The interior of the " reflection absorption chamber " 28, Fig. 1, is an ellipsoid of revolution whose major axis is equal to the signal wave-length. A central sound-absorbing partition 37 is provided, or the two halves of the chamber may be slightly separated so as to leave a central air gap. The dimensions of the chamber may be adjusted by moving the end plugs 38, 39. Alternatively the two halves of the chamber may be paraboloids of revolution separated by a substantial distance. Instead of having discs 34 the tube 29 may simply be open at its end, and a chamber of the type described may be inserted at an earlier point in the chain of filters shown in Fig. 1.